1. Field Of The Invention
The present invention relates to an adhesion measuring apparatus and method for measuring an adhesive force of a sample surface. The present invention also relates to a semiconductor device manufacturing method using the adhesion measuring method.
2. Description of the Related Art
FIG. 20 shows a conventional interatomic force microscope. A laser beam emitted from a semiconductor laser unit 4 is focused on an upper surface of a cantilever 1, and a reflected beam from the cantilever 1 enters a photodiode detector 5. The photodiode detector 5 detects a shift in position of the reflected beam from the cantilever 1, thereby determining a minute flexure of the cantilever 1 due to the interatomic force acting between a probe 2 provided at the distal end of the cantilever 1 and a sample 3 to be measured.
A description will now be made of operation of measuring an image of surface irregularities of the measured sample 3 by such an interatomic force microscope. First, a voltage is applied to a Z-electrode of a cylindrical piezoelectric element 6 by a controller 7 to perform feedback control while moving the measured sample 3 in the Z-direction (i.e., vertically) so that the reflected beam from the cantilever 1 enters a fixed position on the photodiode detector 5. While the cylindrical piezoelectric element 6 is thus actuated in the Z-direction under feedback control, voltages are applied to X- and Y-electrodes of the cylindrical piezoelectric element 6 by a computer 8 through the controller 7 so as to scan the measured sample 3 in the X- and Y-directions simultaneously. By reading the respective voltages in the X-, Y- and Z-directions applied from the controller 7 to the cylindrical piezoelectric element 6, an image of the sample surface can be produced.
As described in Japanese Patent Application No. 5-26841 previously filed, the inventor has proposed a method of measuring surface adhesion of the measured sample 3 using the interatomic force microscope shown in FIG. 20. The term "surface adhesion" employed herein means an adhesive force between a material making up the sample surface and a material to be formed on the sample surface. The surface adhesion is measured by, for example, moving the measured sample 3 vertically to change the surface position of the measured sample 3 relative to the probe 2, and determining a flexture of the cantilever 1 with respect to a Z-directional displacement of the measured sample 3 at this time. The flexure of the cantilever 1 with respect to the Z-directional displacement of the measured sample 3 is measured by the photodiode detector 5 as a shift of the position where the laser beam reflected by the cantilever 1 enters the photodiode detector 5.
More specifically, the surface adhesion is measured in accordance with sequential steps S1 to S7 below.
S1: First, the probe 2 is moved to one measuring point on the measured sample 3.
S2: Assume here that an output voltage of the photodiode detector 5 is Vd and an arbitrary set voltage is Vs. A stepping motor (not shown) for moving the cylindrical piezoelectric element 6 in the Z-direction is actuated so that the measured sample 3 comes closer to the probe 2 of the cantilever 1.
S3: When the measured sample 3 reaches a position near the probe 2, a voltage is applied to the piezoelectric element 6 by the controller 7 to move the piezoelectric element 6 in the Z-direction, making the measured sample 3 come further closer to the probe 2. This produces an interatomic force acting between the measured sample 3 and the probe 2 to flex the cantilever 1. The incident position of the laser beam on the photodiode detector 5 is thereby shifted, whereupon the output voltage Vd of the photodiode detector 5 is varied. When the offset voltage represented by the sum Vd+Vs of the output voltage Vd and the set voltage Vs becomes 0, a feedback circuit in the controller 7 is turned on to apply a voltage Vz to the Z-electrode of the piezoelectric element 6 from the controller 7 for automatic control so that the offset voltage is maintained at 0. The voltage Vz applied in such a feedback position is assumed to be Vc.
S4: The feedback circuit in the controller 7 is turned off.
S5: A triangular wave of .+-.160 V with the applied voltage Vc at the center is additionally applied to the Z-electrode of the piezoelectric element 6 to move the measured sample 3 up and down in the Z-direction. The flexure of the cantilever 1 with respect to the Z-directional displacement of the measured sample 3 at this time measured by the photodiode detector 5 is read from an output voltage value of the photodiode detector 5. Graphic representation of the dependency of the offset voltage Vd+Vs upon the voltage Vz applied to the piezoelectric element is called a Force-Curve.
S6: The feedback circuit in the controller 7 is turned on again for moving the measured sample 3 in the Z-direction to the original feedback position.
S7: The above steps S1 to S6 are repeated several times for one measuring point.
The Force-Curve obtained as described above is shown in FIG. 21. Conditions of the cantilever 4 in points A to G on the Force-Curve of FIG. 21 are shown in FIGS. 22A to 22G, respectively. In FIG. 21, the vertical axis represents the offset voltage Vd+Vs, i.e., the force acting between the probe 2 and the measured sample 3. At a certain position in the direction of the vertical axis, F=0. A repulsion is produced in a region on the positive side in the direction of the vertical axis from F=0, whereas an attraction is produced in a region on the negative side in the direction of the vertical axis from F=0. The larger the distance from the straight line indicative of F=0, the stronger will be either force. On the other hand, horizontal axis represents the voltage Vz applied to the Z-electrode of the cylindrical piezoelectric element 6. The measured sample 3 and the probe 2 of the cantilever 1 come closer to each other with a point on the curve moving toward the left in FIG. 21.
First, at the point A on the straight line of F=0, no forces act between the cantilever 1 and the measured sample 3 as shown in FIG. 22A. When the voltage Vz applied to the piezoelectric element 16 is gradually increased to make the sample 3 come closer to the cantilever 1, an attraction abruptly acts on the cantilever 1 at the point B in FIG. 21 because the probe 2 absorbs a layer of contaminants such as moisture on the surface of the sample 3, i.e., a so-called contaminant layer 3a. Therefore, the probe 2 of the cantilever 1 comes to a position closest to the sample 3 as shown in FIG. 22B. When the sample 3 is further raised in the Z-direction, the attraction acting between the probe 2 and the sample 3 is diminished, resulting in F=0 at the point C. After that, a repulsion acts between the probe 2 and the sample 3. Thus, the warping of the cantilever 1 is canceled at the point C as shown in FIG. 22C, and the cantilever 1 is then curved in the direction of parting the probe 2 from the sample 3 at the point D as shown in FIG. 22D.
Under the above condition, when the voltage Vz applied to the piezoelectric element 16 is now gradually reduced to displace the sample 3 farther away from the cantilever 1, the repulsion is also diminished correspondingly, resulting in F=0 at the point E where the warping of the cantilever 1 is canceled, as shown in FIG. 22E. When the sample 3 is displaced even farther away from the probe 2, an attraction acts between the two members. The attraction is gradually increased, causing the cantilever 1 to warp toward the sample 3 as shown in FIG. 22F. Reaching the point F, however, there occurs an abrupt jump from the attraction region to the point G, whereupon the probe 2 of the cantilever 1 is detached from the contaminant layer 3a of the sample 3 so that the cantilever 1 takes a linear shape substantially free from any warping, as shown in FIG. 22G.
The surface adhesion between the sample 3 and the probe 2 is measured quantitatively from the following equation based on the flexure of the cantilever 1 which corresponds to the variation .DELTA.Vz in the voltage Vz applied to the piezoelectric element 6 between the point E indicative of F=0 and the point F in the Force-Curve obtained as above: EQU Surface adhesion=spring constant.times.flexure of the cantilever
Because the Force-Curve represents the interatomic force acting between atoms in the surface of the probe 2 and atoms in the surface of the measured sample 3, the resulting Force-Curve is different depending on materials of the probe 2 or the measured sample 3. In Jpn. J. Appl. Phys., Vol. 32 (1993) L295, for example, two typical Force-Curves C1 and C2 measured by using the conventional interatomic force microscope are depicted as shown in FIG. 23. These Force-Curves C1 and C2 are obtained by measuring the same sample surface using two probes whose surfaces are made of different materials from each other. It is seen that the surface adhesion between the sample and the probe varies depending on the difference in material of the probe surface even with the sample being the same.
As described above, it has been proposed in Japanese Patent Application No. 5-26841, now Japanese Published Document No. 6-241777 to determine the surface adhesion between a probe and a measured sample from measurement of the Force-Curve. But while the specific purpose of the interatomic force microscope is to produce an image of surface irregularities for determining three-dimensionally, the shape of the sample surface, the surface adhesion is only considered as a physical quantity that is determined depending on a material of the sample surface and a material of the probe. Accordingly, it has heretofore been just proposed to measure the Force-Curve at one arbitrary point on the measured sample surface, thereby determining the surface adhesion.
However, when a multilayer structure is formed through a number of processes as needed in, for example, general semiconductor devices, residual particles due to the preceding process sometimes exist on the surface of a certain layer. In such a case, with an image of surface irregularities, the shape of the layer surface can be determined, but whether foreign matter of different constituent elements exist on the surface cannot be confirmed. Also, the surface adhesion is different between areas where residual particles are present in areas where no residual particles are present because of the different in constituent elements. Accordingly, there is a fear that an accurate adhesion force may not be obtained from a measurement made at only one point.
Thus, the interatomic force microscope and the adhesion measuring method in the prior art have difficulties in accurately determining the condition of the sample surface at an atomic level.